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141.
142.
YuriDZakharov AlexanderSBiakov AymonBaud HeinzKozur 《中国地质大学学报(英文版)》2005,16(2):141-151
Data obtained on conodont distribution in the Permian-Triassic Sovetashen Section of Transcaucasia provide further limitations on the age of the carbon-isotopic anomalies discovered by Baud et al. (1989). The significance of Caucasian sections for working out the carbon-isotope standard for the Upper Permian and Lower Triassic (Induan) is shown. Original data on carbon-isotope composition of bivalve and brachiopod shells from Permian sediments of North-Eastern Russia (Omolon and Okhotsk areas) have been obtained, which may be used for their correlation. 相似文献
143.
U-Th-Pb isotopic data are reported for mineral fractions, individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite Richardton (H5). Chondrules and milligram-sized fractions of pyroxene-rich chondrule fragments contain highly radiogenic Pb and concordant or nearly concordant U-Th-Pb isotopic systems, and are suitable for precise Pb-Pb age determinations. Olivine and sulfide have low U concentrations and contain less radiogenic Pb. The ages of individual chondrules, pyroxene-rich and phosphate fractions are determined using U-Pb and Pb-Pb isochron and model date calculations. The Pb-Pb isochron date of 4562.7 ± 1.7 Ma of the Richardton chondrules and chondrule fragments is resolved from the Pb-Pb isochron date of 4550.7 ± 2.6 Ma obtained from multiple phosphate fractions. Possible biases of the isochron dates due to single-stage approximation of multi-stage evolution, contamination with modern common Pb, and disturbance to the system by reheating, are examined and are found to be insignificant. The chondrule and phosphate dates are interpreted as the timing of cessation of Pb diffusion during cooling following metamorphism in chondrite parent bodies. The difference in estimated closure temperatures, ∼950-1150 K for pyroxenes, and 700-800 K for phosphates (temperature estimates are based on published diffusion rates for Pb in pyroxenes and apatite), allows evaluation of the average cooling rate at 26 ± 13 K/million years for the Richardton parent body over the period of 4563-4551 my. Thermal modeling of the H-chondrite parent body (which is assumed to be asteroid 6 Hebe, heated by decay of 26Al) suggests a scenario in which accretion initiated at 1.7 m.y. after formation of calcium-aluminum-rich inclusions and continued for 3.5 m.y. 相似文献
144.
145.
On the basis of our analysis of a number of continuous observations made by the monitoring network on Crete, we assume that
electrically active clouds are produced in an atmosphere above the sea on the eve of earthquake. These electrically active
clouds, which occur at heights of 0.1–10 km, create the conditions for electrical discharges in an atmosphere that may be
the source of the very high frequency (VHF) radio-emissions registered on Crete. We further suggest possible mechanisms of
thunder electricity generation. We present the model of convection transport in which the first condition in the generation
of thunder electricity is an atmosphere with a horizontal gradient of temperature. Base on this model, the occurrence of electrical
charges on the surface of the sea and their transportation further upwards to heights of up to 10 km is due to pollution energy
allocated within the bottom of the sea as gases and heat injection. The average flux density and power estimations of the
VHF precursors were made for the Crete net situation to compare with published VHF data and radio star sources. 相似文献
146.
Structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant. Structural softening is fundamentally different than material softening, where the decrease in stress is generated by a change in rheological properties with progressive deformation, such as grain size reduction resulting from large shearing strain. We study structural softening generated by folding of the crust-mantle boundary, which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere. For ductile rheologies, the stress decrease represents a decrease of the effective lithospheric viscosity, which is proportional to the ratio of stress to lithospheric shortening strain rate. We present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate. The decrease in effective viscosity can be up to 10-fold. 相似文献
147.
Nd and Sr isotopic data are presented for the 2449–2441 Ma Olanga and Burakovka layered mafic complexes in the eastern Baltic
Shield. These complexes have similar tectonic position, but differ in two aspects: the age of the enclosing crust and the
post-crystallization metamorphic history. The Sm–Nd isotopic results for the Kivakka and Lukkulaisvaara intrusions, Olanga
Complex, are consistent with the model of closed-system crystallization of a single magma without significant wallrock assimilation.
The Rb–Sr systems of minerals were disturbed by late Rb addition during 1.75–1.50 Ga metamorphism. The Nd and Sr isotopic
systems in the Burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct
components. Isotopic variations in the Burakovka Complex can be explained by a 4–20 per cent contamination of a primary komatiitic
or picritic magma with a Mesoarchean crust, similar to that exposed in the region. A similar model, applied to the Olanga
Complex using a Neoarchean crustal isotopic composition, cannot reproduce the observed isotopic signature. The nearly uniform
initial ɛNd values between −1 and −2.3, observed in the Kivakka and Lukkulaisvaara intrusions of the Olanga Complex, as well as in the
other 2.50–2.44 Ga layered mafic intrusions throughout the eastern Baltic Shield, are better explained by a mantle plume model
with small amounts of crustal contamination and minor involvement of asthenospheric material. This model is also consistent
with the geological observations and the temporal distribution of the Paleoproterozoic mafic magmatism in the eastern Baltic
Shield. As an alternative, the enriched isotopic characteristics may be explained by melting of a metasomatically modified
lithospheric mantle source.
Received: 4 August 1994/Accepted: 5 April 1996 相似文献
148.
Carbonate-bearing mantle peridotite xenoliths from Spitsbergen: phase relationships,mineral compositions and trace-element residence 总被引:11,自引:0,他引:11
D. A. Ionov Suzanne Y. O’Reilly Yuri S. Genshaft Maya G. Kopylova 《Contributions to Mineralogy and Petrology》1996,125(4):375-392
Carbonates of mantle origin have been found in xenoliths from Quaternary basaltic volcanoes in NW Spitsbergen. The carbonates
range from dolomite to Mg-bearing calcite and have high Mg-numbers [Mg/(Mg+Fe)=(0.92–0.99)]. In some samples they occur interstitially,
e.g. at triple junctions of silicate minerals and appear to be in textural and chemical equilibrium with host lherzolite.
Most commonly, however, the carbonates make up fine-grained aggregates together with (Ca,Mg)-rich olivine and (Al,Cr,Ti)-rich
clinopyroxene that typically replace spinel, amphibole, and orthopyroxene as well as primary clinopyroxene and olivine. Some
lherzolites contain amphibole and apatite that appear to have formed before precipitation of the carbonates. In situ analyses by proton microprobe show very high contents of Sr in the clinopyroxene, carbonates and apatite; the apatite is
also very rich in LREE, U, Th, Cl, Br. Disseminated amphibole in carbonate-bearing rocks is very poor in Nb and Zr, in contrast
to vein amphibole and mica from carbonate-free rocks that are rich in Nb and Zr. Overall, the Spitsbergen xenoliths provide
evidence both for the occurrence of primary carbonate in apparent equilibrium with the spinel lherzolites (regardless of the
nature of events that emplaced them) and for the formation of carbonate-bearing pockets consistent with metasomatism by carbonate
melts. Calcite and amorphous carbonate-rich materials occur in com- posite carbonate-fluid inclusions, veins and partial melting
zones that appear to be related to fluid action in the mantle, heating of the xenoliths during their entrainment in basaltic
magma, and to decompression melting of the carbonates. Magnesite is a product of secondary, post-eruption alteration of the
xenoliths.
Received: 6 October 1995/Accepted: 17 June 1996 相似文献
149.
Yuri E. Litvinenko 《Solar physics》1996,167(1-2):321-331
A new model is presented for the distribution of solar (and stellar) flare energies. Flares are assumed to result from energy release in multiple reconnecting current sheets (RCSs) in the solar corona. The model takes into account both the dynamical evolution of separate RCSs and their interaction by virtue of coalescence. Thus an attempt is made to generalize previously suggested avalanche and RCS models for the flare frequency distribution. The number of RCSs is not specified and the strength of their interaction is governed by a coalescence parameter. Under quite general assumptions, the resulting distribution can be approximated by a power law with the slope 3/2 < < 7/4, as required by numerous observations. A softer distribution of numerous nanoflares, responsible for the coronal heating, is also possible under suitable choice of parameters. 相似文献
150.